With the advent of the information society, online information exchange, information search, remote services, home office and telephone conferences and entertainment activities such as watching movie and TV etc. have gradually become a part of the modern life, which greatly promoted the broadband access technique to develop in a faster and wider direction. The Passive Optical Network (PON) is one of the widest, fastest and most environment-friendly broadband access techniques and is being accepted by the vast majority of operators for deploying optical fiber access systems so as to gradually replace the existing copper wire (wired) systems, that is, the trend of optical fiber in copper wire out, for meeting the growing communication users and faster and better service requirements.
The Passive Optical Network (PON) is a point-to-multipoint fiber access technique, as shown in FIG. 1. The passive optical network comprises an Optical Line Terminal (OLT), an Optical Network Unit (ONU) and an Optical Distribution Network (ODN). Generally, the PON is a point-to-multipoint structure that is composed by one OLT connecting with multiple ONUs via the optical power splitter (the splitter) of the ODN, wherein each ONU includes an Optical Network Terminal (ONT) for Fiber To The Home (FTTH), and the ONT is a special form belonging to the ONU definition areas.
In the process of deploying the passive optical network, the operators consider how to meet the growing customer demands for the FTTH and save the investment costs as much as possible. Therefore, it expects that one OLT can drive as many optical network units (ONUs/ONTs) as possible via the ODN, which is required that the splitter in the ODN should increase its splitting ratio. However, although the ONUs would be doubled at each time when the splitting ratio increases, the optical link loss will correspondingly increase 3 dB. Theoretically, the loss of the 1:2N splitter is 3*N (the unit is dB), but due to the imperfection of a plurality of production processes and factors such as the fiber coupling loss, the practical loss is 3*N+M (in dB). The M value of the 1:2N splitter having the best or minimum loss is generally less than 3 dB. The largest 1:2N splitter supplied on the market is 1:64, that is, N=6. The largest splitter can be provided by most of manufacturers is 1:32, that is, N=5. Therefore, a splitter whose N>6 must be a combination of smaller splitters. The combination may be varied and can have many levels. However, the more the levels are, the more the processes are, and the larger the corresponding loss is. A 1:2N splitter with the minimum loss generally has two levels at most.
FIG. 2, FIG. 3 and FIG. 4 show the best configuration combinations of splitters whose splitting ratios are 1:128, 1:256 and 1:512 respectively, the corresponding losses are 3*(N+1), that is, 24 dB, 27 dB and 30 dB. If a 5 km optical distribution network is taken as an example, considering that the loss per kilometer of the fiber is 0.4 dB/km and neglecting other losses, the minimum optical link losses relative to these three splitters are 26 dB, 29 dB and 32 dB. Obviously, the loss of the splitter in the entire optical link loss has the largest proportion, thus selecting a splitter with fewer levels plays a vital role of reducing the loss of the entire optical link in the ODN.
For rural areas or rural-urban fringe areas, since the population density is sparse and scattered, operators hope to have a long distance passive optical network to meet their needs very much, and the distance is in the range of 20 km˜60 km. Usually splitters whose splitting ratio is 1:16 is selected. If the fiber loss per kilometer is 0.4 dB/km and other losses are neglected, the losses of 20 km, 40 km and 60 km optical link are 23 dB, 31 dB and 39 dB. Since the long distance PON has a huge optical link loss, the general practice is to add a long-distance box for optical amplification or optical reset in the trunk optical fiber, but such an approach is contrary to the original intention of the passive, that is, it needs to supply power to the long-distance box in the transmission, and thus the operators concern whether there is possibility of meeting long distance requirements without the long-distance box.
No matter whether the passive optical network is the large splitting ratio passive optical network or the large distance passive optical network, the essence is that their optical link losses are enormous, and in order to enable them to run in the optical distribution network, besides setting the best splitter to reduce losses, the corresponding devices for an optical power budget must be provided for them, that is, their optical power budgets must be able to overcome the minimum optical link losses so that the system can run. However, in the existing passive optical network standards, only a few standards such as: IEEE P802.3av (10G Ethernet Passive Optical Network) can provide more than 30 dB optical power budget categories, namely, PR30 and PRX30, as shown in Table 1.
TABLE 1IEEE P802.3av optical power budget categoriesPR10PR20PR30PRX10PRX20PRX30Downlink (dB)22.525.530.522.525.530.5Uplink (dB)232732232630.4
It can be analyzed from Table 1 that the PR30 or PRX30 optical power budget can only support the less than 10 km distance passive optical network with the best 1:2N (N=7) splitter (that is, the splitting ratio is 1:128), however for an optical distribution network having the larger splitting ratio 1:2N (N>7) or over the 40 km long distance, the PR30 or PRX30 optical power budget is far from enough. Therefore, in order to support the large splitting ratio and long distance passive optical networks, it needs to provide categories with the larger optical power budget and the relevant devices.